Out of my last 2x grows, 1 of 3 plants have dwarfed like this. I had an Amnesia auto do it, and this time a Gorilla Cookies is doing it. I have another Gorilla Cookies growing normal right next to it, sprouted at the same time. 1st pic is of the Amnesia auto, as soon as I topped it, it went straight into flowering, no vegging at all. Got 11 grams dried out of the entire plant. But! it was by far the best Sativa I myself have personally smoked in over 30 years. It seemed with fewer budsites to supply, it just got super strong for the few it had. It was done in 60 days, 99% milky trichomes, 1% amber. It was a mind warping high.
The 2nd pic below is my Gorilla Cookies doing the same thing, except I didn't top it, and I didn't stress it at all. I'll get about the same eight from this one of about 10g, I'm just hoping its potent like the Amnesia was.
3rd Pic is of the same GC seeds (back right) small, but growing normally, sprouted at the same time as the dwarfed one, raised in the same tent, same nutes, same conditions as the other plant.
The Gorilla Cookies (back right) vegged a lot more, I didn't do any LST on it either, I wanted to know if I was stunting them, so I'm letting them grow free this time. The plant to the Left is my Durban Poison/BlueBerry, I coiled that one like a snake, and it's doing very well.
The only thing that I can settle on that is making my autos go random dwarfs. Is my temps, I cannot get a stable temp in my grow room, it goes from 68f-82f quite often. The humidity is easy, I stay at 80% for seedlings, 70-75% vegging, and drop it below 50% for flowering.
Autos are quick, but they're a pain in the arse too. You get zero time to fix mistakes before it hurts your yield. I started growing them thinking they were easier, but you need to have a rigid plan with autos, and be meticulous to get the best out of them. After my auto seeds are done, I switching to Photos where you're more in control and have time to correct mistakes.
So if you're temps are constantly going up and down like mine, this could maybe be your issue too?